The invention relates to an exposure station for a photocopier wherein different enlargement sizes can be set and the luminance conditions of the copy light falling on the original can be adjusted.
Today the processing of photographic film, particularly negative film, and the production of the corresponding pictures is no longer perforated exclusively in large photo laboratories, but frequently occurs in a decentralized fashion. To an increasing degree, small photo and home electronics shops, pharmacies, and departments within supermarkets develop negatives and provide pictures of the desired format. Since the problem of available space is usually a consideration in these decentralized locations, compact processing equipment is desirable. At the same time, the processing equipment should permit diverse application and make it possible to prepare different picture formats from negatives that are often of a small film format.
In the optical reproduction system of a photocopier the illumination strength in the picture plane is inhomogeneous given a precisely homogeneous illumination of the transparent original. From the center of the picture to the rim of the picture, the strength of illumination decreases according to a cos.sup.4 law of the picture angle. The picture angle is defined as the angle between the line defined by a point on the negative and the corresponding picture point on the picture plane and the optical axis. In known copiers the distance between the negative and the picture plane is usually relatively large and ranges from about 500 mm to about 800 mm. For these relatively large distances there are generally relatively small picture angles. In addition, large devices, particularly high capacity printers, are usually not equipped to deal with a large number of different picture formats, since their purpose is mainly the rapid processing and production of a large number of pictures of a single format. In these copiers the same maximum picture angle occurs due to the fixed size of enlargement. For such copiers, the problem of rim diminution is consequently a "static" one, which can be resolved by a suitable design of the lens system, for example with a condenser. This achieves a controlled degree of rim enhancement for the illumination strength, which is intended to compensate for rim diminution. Copiers are also known that are suited for differing picture formats. In these devices, interchangeable condensers are provided which are intended to accommodate different degrees of rim diminution. However, interchangeable condensers, and even illumination units, are relatively expensive and are sometimes also large in design.
In the compact copiers two factors contribute to increasing the difficulty of rim diminution. The demanded compactness of the copier requires that the distance between the negative and the picture plane be reduced, for example, to about 400 mm. At the same time, the compact photocopier should be able to produce pictures of 3.5".times.5"to 6".times.9" format from negative originals. For originals of the small 24 mm.times.36 mm format, where the distance between the focusing lens and the original ranges between 40 mm for a large enlargement to 80 mm for a small enlargement, the naturally arising diminution in illumination strength at the picture rim is 40% and 13%, respectively. The correct picture exposure of an original requires a rim diminution of only 5% to 15%, however. If these limit values are exceeded in either direction, the different illumination strengths become visible in the center and edge of the picture and the result is a picture that creates the impression of incorrect lighting during picture-taking.
In contrast to high capacity printers, the problem of rim diminution in the illumination strength arises in the compact copiers as a "dynamic" problem. Thus, the problem of rim diminution in the illumination strength remains to be solved in photocopiers, particularly those of compact design. Specifically, it should be possible to compensate in a controlled fashion for the differing degrees of rim diminution for different picture formats or enlargement sizes and to allow the copier to quickly adjust for the changed conditions. The solution should be simple and permit a savings in space. In addition, it should be cost effective in implementation.